[fpc-devel] Proposal for implementing named address space support

[Florian Klämpfl]

Am 08.01.21 um 22:00 schrieb Christo Crause via fpc-devel:
> On Sun, Oct 4, 2020 at 1:49 PM Christo Crause <christo.crause at gmail.com 
> <mailto:christo.crause at gmail.com>> wrote:
> 
>     FPC can use the section modifier to specify which address space
>     should be used for data.
> 
> 
> I've made some progress in certain areas (basically static variables), 
> the following type of AVR code example works:
> var
>    w: word = $BEEF; section '.eeprom';
>    b: byte;
> begin
>    b := hi(w);
> end.
> 
> However I see problems with propagating section information through 
> reference type parameters. I'm not sure how to propagate the section 
> information of the variable "w" through the parameter into the procedure 
> of the following example:
> var
>    w: word = $BEEF; section '.eeprom';
>    b: byte;
> 
> procedure doSomething(constref tmp: word);
> begin
>    b := hi(tmp);
> end;
> 
> begin
>    doSomething(w);
> end.
> 
> At the moment there doesn't appear to be a way to do this without 
> changing or adding something to the compiler.  One idea could be to 
> define new types identifying types in a specific address space, such as 
> Teepromword. In this case the above example could be written as:
> var
>    w: Teepromword = $BEEF;
>    b: byte;
> 
> procedure doSomething(constref tmp: word); overload;
> begin
>    b := hi(tmp);
> end;
> 
> procedure doSomething(constref tmp: Teepromword); overload;
> begin
>    b := hi(tmp);
> end;
> 
> begin
>    doSomething(w);
> end.
> 
> This way the compiler knows how to generate appropriate access based on 
> the parameter type. It however leads to lots of code duplication.  Which 
> got me thinking about generics - could this be handled internally in the 
> compiler via generics?  It isn't clear to me how to tackle these issues 
> so any advice would be appreciated.

I would simply not allow to use such references as independent pointers. 
This causes too much bloat. They can be used in assignments and 
expressions and that's it.